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A 8 day exposure to NaCl reduced the length of root apical meristem in Secale cereale (cv. K2) to a greater extent than Triticum aestivum (cv. Chinese Spring). NaCl induced also root hair formation much closer to the root cap boundary than in the control roots. Contrasting cultivars were differentially sensitive by NaCl in respect of distance of the first root hair from the root cap boundary in root meristem and this may be used as a character for screening genotypes at the seedling stage for variability in NaCl tolerance.

Seedings of Triticum aestivum cvs. Lyallpur-73, Pak-81 and Lu-26-S were grown in solution culture at varying concentration of NaCl for 8 days. Increasing concentrations of NaCl reduced the size of apical meristem and caused the most distal root hair to form closer to the root tip in all the cultivars. Although the position of the first root hair from the root cap boundary was reduced by salinity, yet there were comparatively little differences between these Pakistani Wheat cultivars in response to NaCl, thus in these studies the relative positions of root hairs is not a good discriminating character for assessing the salt tolerance.Keywords: Triticum aestivum, cultivar, NaCl, Root hair, Root cap Introduction An inhibitory effect of toxic metals such as zine on root growth is well documented (Woolhouse, 1983) indeed comparisons of root growth in solutions containing various concentrations of metals with that in control solutions is a long-established method for determining the metal tolerance of plants (Wilkins, 1978) and both interspecific and intraspecific variation in zinc tolerance has been demonstrated. There is however, very little published information on the physiological basis of the inhibition of root growth by zinc. Considerable work have been done on different aspects of salinity yet limited information is available on specific affects of varying salt concentration on plant physiological processes. Root growth which is an integrated process involving cell division, cell extension and cell differentiation, in particular, we know little about the effects of salinity in the root meristem. As an understanding of physiology of the salt tolerance of plants is important for an effective approach to the salinity problems. To understand the mechanisms of salt tolerance control of Pakistani indigenous cultivars would be extremely valuable to overcome the salinity problem in Pakistan, by the development of salt resistance crops. Considerable work has already been done on the chosen Pakistani cultivars, instead of selecting altogether new cultivars, it was considered more useful to investigate these available Pakistani wheat cultivars. Furthermore, the wide genotypic variations between salt tolerant and salt sensitive cultivars, in respect to a number of pertinent physiological and biochemical parameters, have drawn attention to gain insight of the mechanism of salt tolerance by provision of interspecific variability for salt tolerance. Here some effects of NaCl on the size of meristem and relative positions of the first visible signs of the formation of roots hairs are reported.

The seeds of Hordeum sativum lines B-88079 and B-jou87 were grown in aerated 0.1 Rorison nutrient solution for 72 hours. Then the kinetics of cell cycle in the root meristem were determined by a pulse labelling experiment with low specific activity tritiated thymidine following a 70 hours exposures to three concentrations of NaCI (0, 50 and 100 mM NaCI). Hordeum sativum lines B-jou87 had a longer cell cycle duration than that of line B-88079. The mitotic cell cycle in line B-88079 remained unaffected at 50 and 100 mM NaCI concentrations but that of B-jou87 was decreased. The growth fraction (refers to percentage of meristematic cells which are actively cycling as opposed to these that are not cycling in the meristem) was not determined due to non labelling of any cells in continuos labelling experiment.

The seedlings of Triticum aestivum cvs. Lyallpur 73 and Lu-26-S were grown in aerated 0.1 Rorison nutrient solution for 72 hours. Then the kinetics of cells cycle in the root meristem were determined by a pulse labelling experiment with low specific activity tritiated thymidine following a 70 hours exposures to three concentrations of NaCl (0, 30, 70 mM NaCl). Treatments with 30 and 70 mM NaCl increased the length of cell cycle in cv Lu-26-S but that of Lyallpur 73 was decreased. The growth fraction (refers to percentage of meristematic cells which are actively cycling as opposed to these that are not cycling in the meristem) was increased in Lyallpur 73 from 7.9 to 11.11% over the range of 0-70 mM NaCl whereas that of Lu-26-S was decreased in response to increasing NaCl concentration.